1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a noise reduction method and system of capacitive multi-touch panel.
2. Description of Related Art
The principle of touch panels is based on different sensing manners to detect a voltage, current, acoustic wave, or infrared to thereby determine the coordinate of a touch point on a screen as touched by a finger or other medium. For example, a resistive touch panel uses a potential difference between the upper and lower electrodes to compute the position of a pressed point for determining the location of the touch point, and a capacitive touch panel uses a capacitance change generated in an electrostatic combination of the arranged transparent electrodes with the touching part of a human body to generate a current or voltage for detecting the coordinate of the touching part.
Upon the principle, the capacitive touch technologies can be divided into a surface capacitive and a projected capacitive sensing. The surface capacitive sensing has a simple configuration, so that the multi-touch implementation is not easy, and the problems of electromagnetic disturbance (EMI) and noises are difficult to be overcome. Therefore, the popular trend of capacitive touch development is toward the projected capacitive sensing.
The projected capacitive sensing can be divided into a self capacitance and a mutual capacitance sensing. The self capacitance sensing indicates that a capacitance coupling is generated between a touch object and a conductor line, and a touch occurrence is decided by measuring a capacitance change of the conductor line. The mutual capacitance sensing indicates that a capacitance coupling is generated between two adjacent conductor lines when a touch occurs.
A typical self capacitance sensing senses the grounded capacitance on every conductor line. Thus, a change of the grounded capacitance is used to determine whether an object is close to the capacitive touch panel. The self capacitance or the grounded capacitance is not a physical capacitor, but parasitic and stray capacitance on every conductor line. FIG. 1 is a schematic view of a typical self capacitance sensing. As shown in FIG. 1, during the first time interval, the driving and sensing devices 110 in a first direction drive the conductor lines in the first direction in order to further charge the self capacitance of the conductor lines in the first direction. During the second period, the driving and sensing devices 110 sense the voltages on the conductor lines in the first direction, thereby obtaining m data. During the third period, the driving and sensing devices 120 in a second direction drive the conductor lines in the second direction in order to further charge the self capacitance of the conductor lines in the second direction. During the fourth period, the driving and sensing devices 120 sense the voltages on the conductor lines in the second direction, thereby obtaining n data. Accordingly, there are m+n data obtained.
The typical self capacitance sensing of FIG. 1 connects both a driver circuit and a sensor circuit on the same conductor line in order to drive the conductor line and sense a signal change on the same conductor line to thereby decide a magnitude of the self capacitance. In this case, the advantages include:
(1) a reduced amount of data since the typical touch panel has m+n data in a single image only, so as to save the hardware cost;
(2) a reduced time required for sensing a touch point since an image raw data can be quickly fetched due to only two sensing operations, i.e., concurrently (or one-by-one) sensing all the conductor lines in the first direction first and then in the second direction, for completing a frame, as well as a relatively reduced time required for converting a sensed signal from analog into digital; and
(3) a lower power consumption due to the reduced amount of data to be processed.
However, such a self capacitance sensing also has the disadvantages as follows:
(1) When there is a floating conductor, such as a water drop, an oil stain, and the like, on the touch panel, it causes an error decision on a touch point.
(2) When there are multiple touch points concurrently on the touch panel, it causes a ghost point effect, so that such a self capacitance sensing cannot be used in multi-touch applications.
Another way of driving the typical capacitive touch panel is to sense a magnitude change of mutual capacitance Cm to thereby determine whether an object is close to the touch panel. Likewise, the mutual capacitance Cm is not a physical capacitor but a mutual capacitance between the conductor lines 230 in the first direction and in the second direction. FIG. 2 is a schematic diagram of a typical mutual capacitance sensing. As shown in FIG. 2, the drivers 210 are located on the first direction (Y), and the sensors 220 are located on the second direction (X). On the touch panel, the conductor lines 230 in the first direction, connected to the drivers 210, are also known as driving lines, and the conductor lines 230 in the second direction, connected to the sensors 220, are also known as sensing lines
At the upper half of the first time interval T1, the drivers 210 drive the conductor lines 230 in the first direction and use the voltage Vy_1 to charge the mutual capacitance (Cm) 250, and at the lower half, all sensors 220 sense voltages (Vo_1, Vo_2, . . . , Vo_n) on the conductor lines 240 in the second direction to thereby obtain n data. Accordingly, the m*n data can be obtained after m driving periods.
Such a mutual capacitance sensing has the advantages as follows:
(1) It is easily determined whether a touch is generated from a human body since a signal generated from a floating conductor is in a different direction than a grounded conductor.
(2) Every touch point is indicated by a real coordinate, and the real position of each point can be found when multiple points are concurrently touched, so that such a mutual capacitance sensing can easily support the multi-touch applications.
Also, there are some disadvantages as follows:
(1) A single image raw data has an amount of n*m, which is relatively higher than the amount under the self capacitance sensing.
(2) A one-by-one scanning is operated in a selected direction. For example, when there are 20 conductor lines in the first direction (Y), the sensing operation is performed 20 times for obtaining a complete image raw data. Also, due to the large amount of data, the time required for converting a sensed signal from analog into digit is relatively increased.
(3) Due to the large amount of data, the power consumption is thus increased on data processing.
Whether the self or the mutual capacitance operation is used, the obtained touching data is likely to be influenced by noises, so that errors may appear on determining the position when the capacitive touch panel is touched, resulting in that the sensing resolution of the capacitive touch panel is influenced.
To overcome this problem, in the U.S. Pat. No. 7, 643, 011 granted to O'Connor, et al. for a “Noise detection in multi-touch sensors”, three stimulus waves, each having a different driving frequency, are output by a manner of mutual capacitance, so as to responsively obtain three touch images, and one with the minimum noise is selected from the three touch images. Thus, the driving frequency corresponding to the touch image with the minimum noise is selected as the active frequency for extracting the touch image and calculating the coordinate or position.
However, in actual application of the mutual capacitance sensing, when there is a touch on the touch panel, the sensing lines involved in the touch position are changed in signals and also influenced by noises from human body, resulting in that the position that is not touched by finger may be erroneously determined to be a touched position.
FIG. 3 schematically illustrates the noise influence on a prior touch panel. As shown, when a finger touches position A of the touch panel, touch noise generated therefrom influences signals on the sensing lines involved in the touched position. That is, during time period Tx, the drivers drive the driving lines, in which voltage Vy_1 is employed to charge the mutual capacitance (Cm). At this moment, the voltage detected by the sensors includes the voltage generated from the finger touching position A of the touch panel and the voltage of the noise caused by the finger touching position A of the touch panel. Due to that the voltage detected by the sensors includes the voltage generated from the finger touching position A of the touch panel, the capability to resist noises is relatively high at this moment.
During time period Ty, the voltage detected by the sensors 220 includes only the voltage of the noise caused by the finger touching position A of the touch panel. At this moment, due to that the voltage detected by the sensors does not include the voltage generated from the finger touching position A of the touch panel, the touch data obtained at position B is likely to be influenced by noises, so that errors may appear on determining the position when the capacitive touch panel is touched, resulting in that the sensing resolution of the capacitive touch panel is influenced. That is, noises caused by finger touching will be distributed and diffused along the direction of the sensing lines.
Therefore, it is desirable to provide an improved noise reduction method and system of capacitive multi-touch panel to mitigate and/or obviate the aforementioned problems.